This invention relates to a loop circuit current level detector, and it relates more particularly to such a detector which operates relatively independently of supply voltage variations within a predetermined voltage range.
It is known in loop circuit monitoring arrangements to use impedance bridge-type circuits with a difference amplifier-type voltage comparator across one bridge diagonal to detect loop circuit closure status. Supply voltage for both the comparator and a loop circuit being monitored is connected across another diagonal of the bridge circuit. Examples of this type of circuit are found in the U.S. Pat. No. 3,622,709 to G. S. Tjaden and U.S. Pat. No. 4,056,688 to K. E. Stiefel. A loop current detector using an unbalanced signal amplifier is shown in the U.S. Pat. No. 3,784,756 to J. M. Nemchik.
A modified voltage difference detecting arrangement is shown in a U.S. Pat. No. 3,748,395 to E. Herter and wherein a diode is connected across one diagonal of a bridge circuit and in a separate series alternating current interrogation circuit which enables a determination of the level of direct current conduction in the diode.
All of the foregoing circuits are either relatively inconvenient to use, e.g., because a separate interrogation circuit is required, or operated with a dependence upon the magnitude of a supply voltage for the detector and for a loop circuit which is being monitored.